WASHINGTON — Lithium batteries that can leak corrosive fluid and start fires have emerged as the chief safety concern involving Boeing's 787 Dreamliner, a problem that apparently is far more serious than government or company officials acknowledged less than a week ago.

The Federal Aviation Administration late Wednesday grounded Boeing's newest and most technologically advanced jetliner until the risk of battery fires is resolved. The order applies only to the six Dreamliners operated by United Airlines, the lone U.S. carrier with 787s. But other airlines and civil aviation authorities in other countries quickly followed suit.

For its part, Boeing said 787s will keep rolling off the assembly line while it works to get the grounded planes flying again.

Only hours before the FAA issued its order, Transportation Secretary Ray LaHood reiterated to reporters that he considers the plane safe and wouldn't hesitate to fly one. LaHood and FAA Administrator Michael Huerta unequivocally declared the plane safe at a news conference last week even while they ordered a safety review of the aircraft.

However, as details emerged of two battery failures only 10 days apart, it became apparent that the FAA wouldn't be able to wait for completion of its safety review before taking action. An inspection of the All Nippon Airways 787 that made an emergency landing in western Japan found that electrolytes, a flammable battery fluid, had leaked from the plane's main lithium-ion battery. Investigators found burn marks around the damage. Japan's Kyodo News agency quoted transport ministry investigator Hideyo Kosugi as saying the liquid leaked through the electrical room floor to the outside of the aircraft.

In the first battery incident on Jan. 7 at Boston's Logan International Airport, it took firefighters 40 minutes to put out a blaze centered in an auxiliary power unit of a Japan Airlines 787. The plane was empty of passengers at the time.

The two incidents resulted in the release of flammable electrolytes, heat damage and smoke, the FAA confirmed. The release of battery fluid is especially concerning, safety experts said. The fluid is extremely corrosive, which means it can quickly damage electrical wiring and components. The 787 relies far more than any other airliner in operation on electronics to function rather than hydraulic or mechanical systems.

The electrolyte fluid also conducts electricity, so as it spreads it can short circuits, interfere with electrical signals and make control of the plane impossible for pilots and ignite fires. And its corrosiveness raises concern about whether a leak might weaken a key support structure of the plane, even though the 787 is the first airliner to be made primarily from lightweight composite materials that are less susceptible to corrosion than aluminum, safety experts said.

“Any time you have leakage of battery fluid, it's a very serious situation,” said Kevin Hiatt, president and CEO of the Flight Safety Foundation in Alexandria, Va., which promotes global airline safety.

Neither GS Yuasa Corp., the Japanese company that supplies the batteries for the 787, nor Thales, which makes the battery charging system, would comment on the recent troubles.